Hybrid nanocellulose material as an adsorbent to remove reactive yellow 2 dye

Textile dyes are frequently disposable in aqueous effluents, making it difficult to remove them from industrial effluents before their release to natural waters. This paper deals with the fabrication of cellulose-based adsorbents by reacting nanocelulose crystalline (nanocel) with N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMSPEDA), forming the hybrid (silylpropyl)ethylenediamine@nanocellulose (SPEDA@nanocel), which was employed as adsorbent for the uptake of reactive yellow 2 dye (RY-2) from aqueous effluents. Characterisation of SPEDA@nanocel was carried out using FTIR, SEM–EDS, XRD, TGA, surface area, pHpzc, and hydrophobicity/hydrophilicity ratio (HI). Also, adsorption studies were thoroughly investigated. The effect of initial pH indicated that the maximum uptake of RY-2 takes place at pH 2, which is an indication of the electrostatic mechanism. The kinetic data carried out with 250 and 500 mg L−1 RY-2 with SPEDA@nanocel followed better the nonlinear fractional-like pseudo-first-order model. The t0.5 and t0.95 for the dye uptake were about 30 and 141 min, respectively. The equilibrium data from 10 to 45 °C indicated that the Liu isotherm model was the best-fitted isothermal model. The maximum sorption capacity attained was 112.3 mg g−1 at 45 °C. The thermodynamic data have shown that the equilibrium was favorable and endothermic, and the ΔH° was compatible with an electrostatic attraction between RY-2 and SPEDA@nanocel. Experiments of desorption of loaded adsorbent showed promising results for real applications since at least 5 adsorption/desorption cycles could be employed without significant changes in the recovery and with high precision.

Hybrid nanocellulose material as an adsorbent to remove reactive yellow 2 dye.

Adsorption experiments
An aliquot of 20.00 mL of Reactive Yellow 2 (RY-2) dye solution with the initial concentration variating from 30.0 to 900.0 mg L -1 was added to 50.0 mL flat-Falcon tubes with 30.0 mg SPEDA@nanocell hybrid material at pH 2.0.The Falcon tubes were capped and disposed of horizontally inside a thermostatic reciprocating agitator (Oxy 350, São Leopoldo,   Brazil).The slurries were shaken at different time intervals between 1 and 240 min at 10° to 45°C with a shaking speed of 120 strikes per minute 39,55,56 .Subsequently, the solid phase was separated from the liquid phase by centrifugation.When necessary, aliquots of 1-10 ml of the liquid phase were diluted to 1.0-25.0mL in calibrated volumetric flasks using the blank solution.The dyes unadsorbed after the adsorption process were measured using the T90+ PG Instruments spectrophotometer at a maximum absorption wavelength of 404 nm (RY-2).
The sorption capacity (Eq 1) and the percentage of adsorbate removed (Eq 2) are given below: %  = 100.( 0 −   )  0 (2) q is the sorption capacity of adsorbate adsorbed by the adsorbent (mg g -1 ).Co is the initial adsorbate solution concentration in contact with the solid adsorbent (mg L -1 ).Cf is the final adsorbate concentration after adsorption (mg L -1 ).m is the mass of adsorbent (g).V is the aliquot of the adsorbate solution (L) introduced in the flask.
The study of the influence of the initial pH of adsorbate was performed at 25°C, using an initial concentration of 300 mg L -1 of RY-2, a time of contact between the adsorbent and adsorbates of 2 h, an adsorbent dosage of 1.5 g L -1 , and pH 2.0-10.0.
The preliminary experiments were conducted to ensure the experimental data's reproducibility, reliability, and accuracy.The relative standard deviations of all measurements were below 3.5% 58,61 .Blanks were run in parallel and corrected when necessary 55 .
The solutions of adsorbates were stored in glass bottles, cleaned, rinsed with deionized water, dried, and stored in a suitable cabinet 55 .
Standard RY-2 solutions (1.0-100.0mg L -1 ) were calibrated in parallel with a blank.The linear analytical calibration curve was performed on the UV-Win software of the T90+ PG Instruments spectrophotometer.The detection limit of RY-2 was 0.28 mg L -1 , with a signal/noise ratio of 3 55 .
A 5.0 mg L -1 of standard RY-2 solutions was used as quality control after every ten measurements to ensure the accuracy of the analytes measurements 55 .
The kinetic and equilibrium data's fitness was done using nonlinear methods, which were evaluated using the Simplex method first and secondly by the Levenberg-Marquardt algorithm using the fitting facilities of the Microcal Origin 2021 software 58 .The suitableness of the kinetic and equilibrium models was evaluated using the residual sum of squares (RSS), the determination coefficient (R 2 ), the adjusted determination coefficient (R 2 adj), the standard deviation of residues (SD), and the Bayesian Information Criterion (BIC) 58,61 .Equations 3 to 7 are the mathematical expressions for respective RSS, R 2 , R 2 adj, SD, and BIC.

𝑛𝑛 𝑖𝑖
(3) In the above equations, the qi, model is the individual theoretical q value predicted by the model; qi, exp is individual experimental q value;  , is the average of all experimental q values measured; n is the number of experiments; p is the number of parameters in the fitting model.
The values of R²adj, SD, and BIC will be presented to compare different models of kinetics and equilibrium presented in this work.The best-fitted model would present R²adj closer to 1.000, lower values of SD, and BIC values.However, the kinetic and equilibrium models could not merely be chosen based on the values of R² 58 when these models present a different number of parameters.Therefore, it is necessary to check if the improvements in the R² values are due to the increase in the parameters 58 or if, physically, the model with more parameters better explains the process taking place 58 .
However, the difference in BIC values between models could be conclusive if the difference in BIC values ≤ 2.0) shows no significant difference between the two models 58,61 .
When the difference in BIC values is between 2 and 6, there is a positive perspective that the model with lower BIC is the most suitable 58,61 .For variations of BIC values from 6-10, there is s4 a strong possibility that the model with a lower BIC value would be the best model to be fitted 58,61 .However, if the difference in BIC values ≥ 10.0, it can be predicted with accuracy that the model with a lower BIC value is better fitted 58,61 .
Langmuir, Freundlich, and Liu's models were employed to analyze equilibrium data.
Where qe is the adsorbate amount adsorbed at equilibrium (mg g -1 ); Ce is the adsorbate concentration at equilibrium (mg L -1 ); Qmax is the maximum sorption capacity of the adsorbent (mg g -1 ); KL is the Langmuir equilibrium constant (L mg -1 ); KF is the Freundlich equilibrium constant [mg.g -1 .(mg.L -1 ) -1/nF ]; Kg is the Liu equilibrium constant (L mg -1 ); nF and nL are the exponents of Freundlich and Liu model, respectively, (nF and nL are dimensionless).
Thermodynamic studies for the adsorption of RY-2 dye onto SPEDA@nanocell were performed at a temperature ranging from 10ºC to 45°C (283 to 318K).
The combination of Equations 15 and 16 leads to equation 18 The nonlinear form of equation 18 is 60 : Where R is the universal gas constant (8.314J K -1 mol -1 ); T is the absolute temperature (Kelvin);   0 is the thermodynamic equilibrium constant, which was calculated according to equation 16.   0 is dimensionless.
0 is calculated by converting the values of Kg (Liu equilibrium constant) or KL (Langmuir equilibrium constant), which is expressed in L mg -1 into L mol -1 .Firstly, the value Kg or KL is multiplied by 1000 (mg g -1 ), and then multiplied by the molecular weight of the adsorbate (g mol -1 ) and by the standard concentration of the adsorbate (1 mol L -1 ) and divided by the activity coefficient of the adsorbate (dimensionless) 58,60 .The solution is assumed to be sufficiently diluted to consider that the activity coefficient is unitary.Making these calculations,   0 becomes dimensionless 58,60 .
Equation 19 was used for calculating ∆H° and ∆S°, and Equation 16 was used for calculating ∆G°.
Structural formula of Reactive Yellow 2 (RY-2; 872.947 g mol ; C H N O S Cl Na ).pK values are given.(b) The optimized 3D structural of RY-2.The dimensions of the chemical molecule and physical-chemical properties were calculated using MarvinSketch version 24.1.2. 3 Dipole Moment 281.79 Debye; van der Waals volume 566.32 Å ; Polar surface area 322.24Å²; Van der Waals surface area (3D) 865.96Å; HLB 36.80.

Fig S6 .
Fig S6.Nonlinear van´t Hoff curve for determination of thermodynamic adsorption parameters for the uptake of RY-2 onto SPEDA@nanocel adsorbent.